On the effect of stress on nucleation and growth of precipitates in an Al-Cu-Mg-Ag alloy
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I.
INTRODUCTION
Al-Cu-Mg-Ag alloys with high Cu:Mg ratios show high strength after artificial aging. This can be attributed to the precipitation of very thin, hexagonal-shaped V plates on {111} Al matrix planes, which is stimulated by trace additions of Ag. Alloys based on the Al-Cu-Mg-Ag system have attractive room and high-temperature strength and creep resistance for temperatures up to 120 7C and are superior to 2618 and 2219.[1,2] The behavior under creep conditions is primarily controlled by the thermal stability of the precipitates, i.e., how the precipitate structure is affected by temperature, time, and stress exposure. In the Al-Cu-Mg-Ag alloy, V partially or completely replaces the well-known {001}-type precipitate sequence in Al-Cu-based systems, i.e., Guinier-Preston (GP) zones, Q" and Q', as transition phases before the equilibrium Q (Al2Cu). As V was recently discovered, the structure of this precipitate is still under discussion. Proposed structures include monoclinic,[3,4] hexagonal,[5] oththorhomibic,[6,7] and tetragonal[8] symmetries. Various chemical analyses of V have also been carried out,[7,9–12] revealing composition close to QAl2Cu. Results indicate that the role of Ag and Mg and the requirement of a high Cu:Mg ratio in promoting V is still unclear. Recent Atom Probe Field Ion Microscopy (APFIM) studies by Hono et al.[13] showed that in the as-quenched condition, independent clusters of Cu, Mg, and Ag were present. After 15 seconds at 180 7C, coclustering of Ag and Mg was observed. After aging for 30 seconds at 180 7C, they detected extremely small precipitates containing Ag, Mg, and Cu atoms. They assume that this is a precursor phase for the formation of V. In general, nucleation of precipitates in an age-hardenaB. SKROTZKI, Research Scientist, is with the Institut fu¨r Werkstoffe I, Rhur-Universita¨t Bochum, 44780 Bochum, Germany. G.J. SHIFLET and E.A. STARKE, Jr., Professors, are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. Manuscript submitted February 7, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
ble aluminum alloy can be described by DG 5 2V z DGV 1 A z g 1 V z DGs
[1]
where DG is the Gibbs free energy change for the transformation to a more stable phase, DGV is the volume free energy change for the formation of the precipitate nucleus, V is the volume of the new phase, A is the area of the interface between the matrix and the precipitate, g is the energy of the new surface formed, and DGs is the increase in elastic strain energy per unit volume of precipitate. The elastic strain energy depends on the misfit d between both phases and the elastic constants of the matrix phase. It is this term the present investigation addresses. Under normal aging conditions, i.e., when no applied or residual stresses are present, an even distribution of precipitates should form on all habit planes. If precipitation occurs preferentially on certain habit planes, an anisotropy of strength properties may result. It is well known t
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